1. Field of the Invention
The present invention relates to a system for transmitting two-level picture signals and more particularly to a system such as facsimile transmission, in which picture signal consisting of low and high, i.e. black and white, levels is divided into blocks, each having a predetermined number of picture elements, the pieces of the signals contained in the respective blocks are converted to the corresponding binary codes, and the binary codes are converted to the corresponding signals having different phases or amplitudes.
2. Description of the Prior Art
With a system for transmitting two-level signal, such as facsimile system, the most important things are to decrease the degradation of the quality of the reproduced picture due to code errors during transmission, to increase the speed of transmission and to simplify the structure of the system. As one of the systems for transmitting coded two-level signal is known the run length coding system. In this system, the information of run lengths, i.e. the lengths of continuing white or black (i.e. high or low) levels, is coded to be transmitted. This system has an advantage that the transmission time can be economized, but also has a disadvantage that once a code error is caused the quality of the reproduced picture is so much degraded.
Moreover, a direct transmission system is known in which the two-level signal is transmitted directly through a modulator and a demodulator (referred to also as MODEM) without coding. With this system, even if the transmission error rate is about 10.sup.-.sup.3, the quality of the reproduced picture is allowable in practice, since the picture signal such as facsimile signal, used in this system has a high redundancy. However, this system has a comparatively long transmission time and in order to increase the transmission speed of the MODEM, i.e. bit rate, an expensive device must be employed.
In these systems described above, i.e. the phase shift keying modulation (a kind of phase modulation) system or the multi-level modulation (a kind of amplitude modulation), the two-level signal is divided into blocks each containing a predetermined number of successive picture elements, the pieces of the signal contained in the respective blocks are coded, and the coded signals are converted into the corresponding signals having a predetermined number of different phases or amplitudes. With this known system, however, certain code errors during transmission will still cause a considerable degradation of the quality of the reproduced picture.